5.2.1

• Autotrophs


  Organisms that use light energy or chemical energy and inorganic molecules (carbon dioxide and water) to synthesise complex organic molecules
• Heterotrophs


  Organisms that ingest or digest complex organic molecules, releasing the chemical potential energy stored in them
• Phototrophs


  Organisms that uses energy from sunlight to synthesise organic compounds for nutrition
• Chemoautotrophs


  Organisms which synthesise complex organic molecules using energy derived from exergonic chemical reactions
• Photosynthesis


  • The process whereby light energy from the Sun is transformed into chemical energy and used to synthesis large/complex organic molecules from inorganic substances
• Photoautotrophs and heterotrophs


  Can release the chemical potential energy in complex organic molecules (made during photosynthesis) - this is respiration. They can also use the oxygen for aerobic respiration
• Photosynthesis


  6CO2 + 6H2O (+ light energy) → C6H12O6 + 6O2
• Organisms evolved that could use oxygen for aerobic respiration


  This releases carbon dioxide back into the atmosphere and produces water
• Photosynthesis in plants


  Two-stage process (light-dependent and light-independent) taking place in chloroplasts
• Compensation point


  When photosynthesis and respiration proceed at the same rate, so there is no net gain or loss of carbohydrate
• Compensation period


  The time taken to reach the compensation point, different for different species of plants
• Chloroplast structure


  • Disc-shaped, 2-10µm long
  • Intermembrane space 10-20nm wide
  • Outer membrane permeable to small ions
  • Inner membrane less permeable and has transport proteins embedded in it
  • Many grana, consisting of stacks of up to 100 thylakoid membranes, provide a large surface area for the photosynthetic pigments, electron carriers and ATP synthase enzymes
  • Photosynthetic pigments arranged into photosystems
  • Fluid-filled stroma contains the enzymes needed to catalyse the reactions of the light-independent stage
• Photosynthetic pigments


  Molecules that absorb light energy, each absorbing a range of wavelengths in the visible region and having its own distinct peak of absorption
• Photosystem


  • A funnel-shaped light-harvesting cluster of photosynthetic pigments, held in place in the thylakoid membrane of a chloroplast. The primary pigment reaction centre is a molecule of chlorophyll a. The accessory pigments consist of molecules of chlorophyll b and carotenoids
• Light harvesting in chloroplast membranes

  1. Photon/light energy absorbed by pigment molecules

  2. Electron becomes excited and moves to a higher energy level

  3. Energy is passed from one pigment to another

  4. Energy is passed to reaction centre/chlorophyll a/ primary pigment
• Primary pigments


  • Chlorophyll a absorbs blue and red light, of wavelength of around 450nm
  • P680 - found in photosystem II and its peak of absorption is light at a wavelength of 680nm
  • P700 - found in photosytem I and its peak of absorption is light at a wavelength of 700nm
• Accessory pigments


  • Chlorophyll b absorbs blue and red light, of wavelengths around 500-640nm
  • Carotenoids absorb blue light but reflect yellow and orange light
  • Carotene - orange
  • Xanthophyll - yellow
• The light-dependent stage of photosynthesis takes place on the thylakoid membranes of the chloroplasts
• The light-independent stage of photosynthesis takes place in the stroma of chloroplasts
• Non-cyclic Photophosphorylation


  1. Light energy hits photosystem I and II

  2. The electrons become excited in chlorophyll a and moves to a higher energy level

  3. The electrons are accepted by electron acceptors and the move along the electron transport chain

  4. The electrons lost in photosystem II are replaced by the electrons made in the photolysis of water (forms hydrogen ions and oxygen)

  5. The electrons lost in photosystem I are replaced by the electrons from photosystem II

  6. Protons are pumped across the thylakoid membrane by the flow of the electrons from the stroma to the thylakoid space, down the proton gradient

  7. The hydrogen ions made in the photolysis of water combines with electrons to form hydrogen atoms

  8. The NADP combines with hydrogen to form reduced NADP

  9. As the protons move across the thylakoid membrane by chemiosmosis through ATP synthase, causing photophosphorylation producing ATP
• Cyclic photophosphorylation


  1. Only photosystem I is used (P700)

  2. The excited electrons pass to an electron acceptor and back to the chlorophyll molecule from which they were lost

  3. There is no photolysis of water and no generation of reduced NADP, but small amounts of ATP are made
• Photolysis of water


  2H20 -> 4H+ + 4e- +O2
• Products of photolysis of water

  • Hydrogen ions – used in chemiosmosis to produce ATP
  • Electrons – replace those lost by the oxidised chlorophyll and to reduce chlorophyll a
  • Oxygen – used by the plant in aerobic respiration but much of it diffuses out of the leaves, through stomata, into the air
• Light-independent stage of photosynthesis

  1. Carbon dioxide combines with ribulose bisphosphate (5C), catalysed by the enzyme rubisco

  2. RuBP becomes carboxylated to make a 6C compound

  3. The 6C compound is unstable so breaks down into 2 molecules of glycerate 3-phosphate – carbon fixation

  4. The products from the light-dependent stage are used to reduce and phosphorylate GP into another 3C compound, triose phosphate

  5. The TP is used to regenerate RuBP using ATP, so that the cycle can start again
• Carbon dioxide


  The source of carbon for the production of all large organic molecules
• Products from GP


  • Amino acids
  • Fatty acids (+ glycerol) -> lipids
  • Glycerol (+ fatty acids) Lipids
  • Hexose sugars (e.g. glucose) that can then be made into polysaccharides like starch and cellulose
• Most triose phosphate is recycled to ribulose bisphosphate. 5 out of 6 molecules of TP (3C) are recycled by phosphorylation, using ATP from the light-dependent reaction, to 3 molecules of RuBP (5C)
• Factors affecting photosynthesis


  • Carbon dioxide concentration
  • Light intensity
  • Temperature
• Carbon dioxide concentration


  More CO2 = more CO2 fixation = more GP = more TP = more regeneration of RuBP
• Stomata opening/closing


  The number of stomata that open to allow gaseous exchange leads to increased transpiration = plant wilts if water uptake from the soil cannot exceed water loss by transpiration = stress response = release of a plant growth regulator (abscisic acid) and stomata close = reduce CO2 uptake and reduce the rate of photosynthesis
• Light intensity


  Increase in light intensity = More light energy available to excite more electrons. More ATP and more reduced NADP produced, which can be used in the light-independent stage as sources of hydrogen and energy

  Decrease in light intensity = GP cannot be changed to TP, so GP will accumulate and levels of TP will fall. Lower the amount of RuBP, reducing CO2 fixation and the formation of more GP
• Temperature


  Increase in temperature = little effect upon the rate of the light-dependent reaction (no enzymes involved), however it will alter the rate of light-independent reaction

  Temperature rises above 25oC = oxygenase activity of rubisco increases more than its carboxylase activity increases – photorespiration exceeds photosynthesis. ATP and reduced NADP from the light-dependent reaction are dissipated and wasted = reduces overall rate of photosynthesis

  High temperature = damage proteins

  Increased temperatures = increase in water loss from leaves by transpiration = closure of stomata = reduction in rate of photosynthesis
• Limiting factor


  A factor that prevents a process from increasing any further at its lowest value
• Increasing carbon dioxide concentration

  Increases the rate of photosynthesis
• Increasing light intensity


  Increases the rate of photosynthesis
• Effects of light


  • Causes stomata to open so CO2 can enter leaves - more carbon fixation
  • Trapped by chlorophyll where it excites electrons - involved in photophosphorylation = more ATP
  • Splits water molecules to produce protons - involved in photophosphorylation = more ATP
• Temperature between 0-25°C


  The rate of photosynthesis approximately doubles for each 10°C rise in temperature
• Temperature above 25°C


  The rate of photosynthesis levels off and then falls as enzymes(ribulose) work less efficiently. Proteins (photosystems and electron carriers) also denature. Oxygen more successfully competes for rubisco and stops it from accepting CO2. More water loss from stomata, leading to a stress response in which the stomata close, limiting the availability of CO2
• Photosynthometer


  Used to measure the rate of photosynthesis by collecting and measuring the volume of oxygen produced in a certain amount of time
• Using a photosynthometer


  1. Fill the apparatus with water

  2. Place a cut shoot, end upwards, into a test tube containing the same water that the plant has been kept in and add 2 drops of sodium hydrogen carbonate solution

  3. Place a light source as close to the beaker as possible

  4. Position the capillary tube over the cut end of the plant and pull the syringe plunger so that the bubble of gas collected is in the capillary tube near the scale

  5. Measure the length of the bubble and note it down